Spreadsheet: Remove Clear All button in favor of API

[rolfsmeds]

The Clear All button in the filter dropdown seems like a bad design decision, seeing as how

• The Select All checkbox can be used to toggle all filers off for that columns, and
• An action for clearing all filters in all columns should not be placed in the filter dropdown in an individual column.

I thereby propose that we remove the Clear All button altogether. Developers can call the existing clearAllFilters method to clear all filters on the table and bind that to a button that they place where they see fit.

[vursen]

The following SpreadsheetFilterTable fields and methods will need to be deprecated first:

• protected void initClearAllButtons()
• protected Button createClearButton()
• Map<PopupButton, Button> popupButtonToClearButtonMap
• public static String CLEAR_FILTERS_BUTTON_CLASSNAME